Wireless communication devices that selectively communicate with a target device in a desired direction by using an adaptive array system have already been put to practical use.
The adaptive array system generally comprises: a plurality of antennas; signal processing units, each of which processes either the transmitting or received signals of one of the plurality of antennas; and a directivity processing unit to form a desired directional pattern of the entire antennas by assigning amplitude and phase differences to the transmitting and received signals of each antenna.
Each signal processing unit of the adaptive array system is operated under automatic gain control (AGC) to ensure the dynamic range of the received signals.
An example of such automatic gain control in an adaptive array system is disclosed, for instance, in Japanese Laid-Open Patent Application Publication No. H03-70221. The adaptive array system has a signal processing unit (branch) with respect to each antenna, and performs automatic gain control in a manner that signals obtained by a single, representative signal processing unit are used to uniformly exercise feedback control on gains of the respective signal processing units. According to this technique, the input signal magnitude in the adaptive array is much the same before and after automatic gain control is applied, exerting no influence on the control of the adaptive array.
However, according to the above-mentioned automatic gain control of the conventional technology, a problem remains that, even if feedback control is performed uniformly on the respective signal processing units by using the same signals obtained from one signal processing unit, the gains of the respective signal processing units are not completely standardized due to variations and time-course changes in elements making up each signal processing unit.
In particular, the adaptive array system forms a desired receiving directional pattern by assigning appropriate amplitude and phase differences to the received signals obtained from the respective received signal processing units. And the respective transmitting signal processing units form a transmitting directional pattern that is the same as the receiving directional pattern by assigning, to the transmitting signals, the same amount of amplitude and phase differences at the reception.
Consequently, there is a problem that, when gain deviations exist among the respective received signal processing units, the receiving directional pattern and the transmitting directional pattern become inconsistent if the amplitude differences of the respective received signal processing units are processed as the amplitude differences of the respective transmitting signal processing units.